These studies deal with a group of mutants blocked in ribosome assembly, but which apparently do not involve the structural components of the E.coli ribosome. Several of the mutants show in vitro complementation, both with wildtype extracts and with mutants of other classes. The pre-ribosomal 43S particles accumulated in mutants are converted to 50S particles by these extracts. The nature of this maturation process will be studied by analyzing the in vitro conversion products for their patterns of base methylation, excess RNA oligonucleitides, protein trimming, etc.